Hardcore mode that introduces new aspects of survival into the gameplay: bullets take up room, you have to sleep, drink water, eat, stimpaks heal you slowly, limbs get hurt and other stuff that make it really feel like you are trying to survive in a post-apocalyptic world.More powerful and varied enemies than in Fallout 3, enjoyable side quests and more choices when completing quests.
Fallout 3 has good customizability, but New Vegas introduced gun customization and bullets variation.